We have developed highly specific antisera to the human receptor for epidermal growth factor (EGF). These antisera have been employed to determine directly factors affecting the metabolism of the receptor protein. We have demonstrated, for example, that incubation of cells with EGF leads to a 10-fold increase in the rate of degradation of the EGF receptor molecule. Also, we have shown that antisera that block the binding of 125I-EGF to its receptor inhibit the biological activities of sarcoma growth factor (SGF). The EGF receptor, therefore, is required for the activities of SGF which is thought to be related to the transformed state of tumor cells.